linux-kernel - Re: [PATCH] x86: set PAE PHYSICAL_MASK

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Date:	Thu, 05 Jun 2008 22:14:26 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...nel.org>
CC:	Jan Beulich <jbeulich@...ell.com>, Ingo Molnar <mingo@...e.hu>,
	Stable Kernel <stable@...nel.org>, x86@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit

H. Peter Anvin wrote:
> Jeremy Fitzhardinge wrote:
>>
>> We could have an auction:
>>
>>    Do I hear 46? 47? 48?  50?  52!  Going once, twice, 52 bits!
>>
>> Anyway, we can fix it later in a separate patch.  This is a 
>> change-as-little-as-possible bugfix patch.
>>
>
> It should either be 52 bits or dynamic based on CPUID information.  
> The latter is very expensive.

I'm more concerned that it might not be possible.  I'm trying to think 
how many places have compile-time constants derived from this mask.  
Maybe not too many.

> If there end up being additional control bits assigned in this space 
> we won't use them  since we know the size of the address space (which 
> won't include the control bits) and thus will leave them at zero.

You mean, if new bits appear we can just adjust the mask accordingly to 
avoid them?  And if we don't use them, then they'll be zero?

> It's largely theoretical, since I believe Linux on x86-64 relies on 
> virtual >= physical+N, where I believe N is about 3 bits, and the page 
> table format or page size need to change to support more than 48 bits 
> of virtual address space.

I don't see any relationship between the physical and virtual size.  
Certainly virtual is fixed at 48 bits (4*9+12), but I don't think 
there's any deep reason why physical needs to be within 3 bits.

    J
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